The CDMA or code division multiple access system used for IS-95 is very different to other multiple access schemes used in previous cellular systems. However it offers a number of advantages and as a result has been widely used in many cellular technologies. IS-95, cdma one was a revolutionary system for cellular telecommunications. It paved the way for the other CDMA based 3G systems that were to follow around the globe. It enabled greater levels of spectrum efficiency to be gained while also allowing many other improvements to be introduced. IS-95 itself evolved into the variety of cdma 2000 schemes including 1x, 1x ev-do, and the plans were ultimately to migrate onto a 4G system known as UMB - Ultra-Mobile Broadband.
The IS-95 channels can be split into those for the forward link and those for the reverse link. The IS-95 channels differ between the forward and reverse links as a result of the different requirements and the different way in which the links operate. Also Walsh codes need to be synchronized if they are to remain orthogonal. As the signals transmitted from the mobile stations travel over different distances because of the variety of locations of the mobiles they will all arrive at slightly different times, and hence they will not be synchronized to one another.
Structure of IS-95 channels in the forward link
This is a total of 64 IS-95 code channels that can be accommodated on each RF channel. The IS-95 channels in the forward link are arranged in the following fashion:
- Pilot channel
- Paging channels (up to seven)
- Sync channel
- Traffic channels
The IS-95 channels are associated with different Walsh codes - particular codes being used to support different functions as outlined below:
To generate the final signal, the data from the individual channels is multiplied with the Walsh codes to provide the individual IS-95 forward link channels. The output from this process is then further multiplied with the short PN codes. This provides a means of identifying the sector / cell from which the signal is coming for the handset / mobile.
IS-95 channels in the forward link
The IS-95 forward link channels and their functions and make-up are summarized below:
- Pilot channel (PC): The pilot channel is transmitted as a reference by the base station to provide timing and phase reference for the mobiles, and carries no real data. The "data" carried by the channel is a continuous stream of zeros which is spread by Walsh code zero, which itself a stream of zeros. This is further spread by a pair of quadrature PN sequences. This means that the pilot channel is effectively the PN sequence with its associated offset. A measurement of the signal-to-noise ratio of the pilot channel also gives the mobile an indication of which is the strongest serving sector.
- Paging channels (PCH): This IS-95 channel is used to carry information to enable mobiles to be paged. Data carried by this IS-95 channel includes system parameters, voice pages, SMS and other broadcast messages. It occupies Walsh codes 1 - 7 dependent upon the system requirements. The PCH carries data at either 4.8 or 9.6 kbps - a field in the Sync Channel indicates the data rate being transmitted.
As with other channels there are a number of stages taken to produce the final channel. First the baseband information is error protected. After this the data is repeated if it is at a rate of 4.8 kbps, otherwise it is left as it is. Following this the data is interleaved and then scrambled by the decimated long PN sequence, and finally spread by the Walsh code for the particular channel assignment. In this process the long PN code is itself masked with a code which is specific to the channel being used. In this way the long PN code for Paging Channel 1 (using Walsh Code 1) is different to Paging Channel 4 (using Walsh Code 4).
- Synchronisation channel (SC): This IS-95 channel is used to provide the timing reference to access the cell. This IS-95 channel always uses Walsh code 32. Each base station has a fixed timing offset to reduce the interference between adjacent base-stations.
The Sync channel incorporates an 80 mSsuperframe structure. This is divided into three 26.667 mS frames which correspond to the same length as the short PN sequences. This means that they align with the timing on the Pilot channel.
This IS-95 channel is allocated the least power of the overhead channels in the overall CDMA transmission. The data that is transmitted on this channel includes the system time, pilot PN of the base station, long code state, system ID, and the network ID.
- Forward Traffic Channel (FTC): As the name implies, the Forward Traffic Channel is used to carry voice, user data, and also signalling information.
When carrying voice, the coded voice data does not require a constant bit rate and IS-95 allows the rate of the frames to change dynamically (every 20 ms). When the rate is reduced it reduces the level of interference to other users. The original vocoder specification used a set of rates based on divisions of 9.6 kbps. This is reflected in IS-95A. Later the vocoder was improved to give better voice quality and in IS-95B a vocoder was introduced with a rate set based on 14.4 kbps. The 9.6 kbps rate set was termed RS1 and the second based on 14.4 kbps was termed RS2. However data is always carried at full rate.
Structure of IS-95 channels in the reverse link
The IS-95 channels for the reverse link are quite different to those in the forward link. There are only two basic channels:
- Access channel
- Reverse traffic channel
The way in which these IS-95 channels are structured and assembled is also different. This is because they are generated within the mobile rather than the base station. In terms of the modulation, OQPSK is used where a half chip delay is introduced onto the Q channel of the modulation.
However orthogonal modulation schemes are used. The different mobiles are individually identified by a mask on the long PN code which is based on the Equipment Serial Number (ESN). The long PN code is used to give the final spreading of the data to 1.228 Mcps.
IS-95 channels in the reverse link
The two IS-95 channels in the reverse link are summarized below:
- Access channel (AC): This IS-95 channel is used by the mobile to communicate with the base station when no traffic channel has been set up. This IS-95 channel is therefore used for gaining acesstoth e network, call origination requests and also for sending responses to paging commands that might be sent bythe network.
There can be up to 32 Access Channels on the IS-95 reverse link for each Paging Channel on forward link. Each AC uses the same PN but they are time shifted to enable the mobile to be uniquely identified. Data is sent at 4800 bps in a 20 mS time frame so that each frame contains 96 bits.
- Reverse traffic channel (RTC): Like the Forward Traffic Channel, this reverse link IS-95 channel is used to carry variable rate voice data, user data and signalling.
The structure of the reverse traffic channel is similar to that of the access channel. However this IS-95 also includes a data burst randomizer into which the orthogonally modulated data is fed. The data burst randomizer is the technique used to account for the variable rate voice data is accommodated - it is not possible to use the same techniques used on the forward link because they affect the channel power. This in turn would upset the power control adjustments that need to be made to ensure that all the mobiles are received as close to the same strength as possible.
Although there are only two IS-95 channels on the reverse link, these are all that are needed to carry the required amount of data from the mobile to the base station.